Overpolymers, prepared by first polymerizing a monomer(s) to a given degree of polymerization and/or particle size to form a core, then polymerizing another monomer(s) over the core to form a shell, are known to the art. The polymerization steps can be carried further to yield yet a third layer called an overcoat, and even fourth layers or more are known. The overpolymer can have hard cores (i.e., high glass transition temperature cores having a Tg value of about 20.degree. C. or higher) or soft cores (i.e., cores having a glass transition temperature of about 0.degree. C. or lower). The glass transition temperatures of the shell and overcoat can alternate with respect to the core such that an overpolymer will be comprised of, for example, a hard core, soft shell, and hard overcoat. Relevant U.S. Pat. Nos. disclosing and teaching the preparation of overpolymers are 3,657,152; 3,661,994; 3,793,402; and 3,803,264.
The overpolymers of the present invention are comprised of a hard predominantly acrylonitrile monomer core, a rubbery shell, and a hard overcoat. These overpolymers have particular utility as tougheners for thermosetting molding compounds such as phenolic resins, furans, epoxy resins, and unsaturated polyester resins, and for thermoplastic molding compounds such as polyvinyl chloride, polycarbonates, polystyrene, polymethacrylates, and polypropylene. Patents directed to the use of overpolymers in thermoset and thermoplastic molding compounds are U.S. Pat. Nos. 3,632,679; 3,655,826; 3,661,994; 3,760,035; 3,793,402; and 3,652,722; 3,787,522; 3,833,682; 3,833,683; 3,880,950; 3,887,645; and 3,894,115. The latter group of patents disclose the use of overpolymers in unsaturated polyester thermoset molding compounds.